Well, good question. I think, personally, that you should not put it in as "babysitting". It would be better if you were more specific. If you just go over to a persons house and watch their kid for few hours, maybe you shouldn't put it in. If you are more serious and run sort of a "daycare", maybe put that in. If you really want to make it sound good, just say that you are "good with children". It makes you sound more friendly and mature but have a fun side still inside of you.
Fill it as a work experience and not extracurricular. Baby sitting actually helps one to develop skills and learn a lot of things which could relate directly to the normal day to day life. That skill could be helpful in performing another duty elsewhere that might not necessarily be related to baby sitting itself.